It is well known for railway signalling purposes the rails are divided up into sections, each of which is inserted into a corresponding electrical circuit known as a track circuit.
As a rule, said track circuits have a signal emitter, generally located at one end of said sections of track, and a receiver for said signal, generally located at the other end of the section of track. If no rolling stock is present on the section of track in question, the receiver duly receives the signal transmitted by the emitter, and this reception is interpreted as "line clear" information.
If, on the other hand, rolling stock is present on the section of track, the axles of the locomotive or of the waggons being pulled by it short-circuit the track circuit, so that the receiver no longer receives the signal from the emitter, or receives a very different signal from the one received when the section of track is clear.
This second situation is interpreted as a "line not clear" signal.
The above, in principle, is how the known type of track circuit operates.
It should be pointed out, however, that traction current also circulates in the rails, and that sometimes said current may have harmonics similar, in terms of waveform, frequency and intensity, to the current transmitted to the track circuit by the emitter. Although it is improbable, it could arise that the receiver interprets as a "line free" signal a disturbing current which is part of the locomotive's traction current, and this is incompatible with the safety conditions demanded of a railway signalling system.
A previous invention by the same Owner (Italian patent no. 1.186.871), exploits the hypothesis of the disturbing signal from the traction current having characteristics of stability over a sufficiently long period of time, and therefore provisions are made to invert the polarity of the signal transmitted by the emitter with a higher frequency.
In this way it is possible, by means of a relatively simple alteration in the track circuit, to recognize a disturbance current with a frequency, waveform and phase similar to the current generated by the emitter, thus preventing any confusion between the disturbance and the signal, as the probability of this similarity being maintained even in the face of the inversion of the signal is practically null.